The present invention relates to an extrusion type of coating apparatus, and more particularly relates to a coating apparatus which can prevent the occurrence of a thick layer so as to provide uniform layer thickness.
There are various kinds of coating, such as roller coating, gravure coating, slide bead coating, curtain coating, and-extrusion coating. The most suitable type of coating apparatus is used for specific purpose. A magnetic recording medium is obtained when a magnetic coating solution is coated on a flexible support (referred to as a web). For the purpose of coating the magnetic coating solution, the extrusion type of coating apparatus is most suitable because a uniform coating layer can be obtained.
On the other hand, the magnetic recording medium has been rapidly improved recently, so that oxidized magnetic powder of high BET value and barium ferrite material are used, and the viscosity of a coating solution has been increased. Further, there is a great demand for coating a thin layer at high speed in order to improve productivity.
For example, methods for extrusion coating of a magnetic recording medium are disclosed in Japanese Patent Application Open to Public Inspection Nos. 84771/1982, 104666/1983 and 238179/1985. According to the aforementioned methods, a layer of uniform thickness can be obtained, however, the region which can be coated in good condition is relatively small. Therefore, under coating conditions of high viscosity, thin layer and high coating speed, it is difficult to obtain a coated layer of desired quality. That is, especially when the thickness of a coated layer is not more than 30 .mu.m (wet layer thickness), a layer of uniform thickness can not be provided. The reason for uneven layer thickness is related to various factors such as the dimensional accuracy of a coater head, the configuration of an edge, the configuration of a liquid reservoir, the tension of a base, and the property of a coating solution. These factors are complicatedly related to each other.
At present, there is a strong demand for a coating apparatus which is not affected by these factors and can provide uniform layer thickness distribution.